The resonator fiber optic gyro (RFOG) senses rotation by detecting the frequency difference between clockwise and counterclockwise resonance frequencies of a fiber optic resonator. The resonance frequencies of the fiber resonator are probed with a narrow band laser or lasers. The resonator translates an optical frequency to an optical intensity. For example, for the reflection resonator as shown in FIG. 1, the light intensity detected by the photodetectors will be maximum when the optical frequency is away from the resonance frequency. However, when the optical frequency approaches the resonance frequency, the light intensity sharply dips to a minimum at the resonance frequency. Therefore, one method to detect the resonance frequency is to detect the minimum power at the detector. However, this method is not accurate enough for rotation sensing and thus another more precise method is needed.
A very precise measurement of the resonance frequency can be made by employing frequency modulation or phase modulation of the light, or by employing cavity length modulation of the resonator. FIG. 1 shows a prior art RFOG employing cavity length modulation. FIG. 2 shows how the modulation can give a precise measurement of the resonance frequency. Section (a) in FIG. 2 shows a resonance dip that would be observed at a photodetector if the frequency of the light is swept. Section (a) also shows the resonator response to sinusoidal frequency modulation at three different nominal optical frequencies. If the nominal optical frequency is to the right of the center of the resonance dip, then an intensity signal will be present at the photodetector that is at the modulation frequency and in-phase with the modulation. If the nominal optical frequency is to the left of the center of the resonance dip, then an intensity signal will be present at the photodetector that is at the modulation frequency, but 180 degrees out-of-phase with the modulation. For an ideal resonator and modulation, when the nominal optical frequency is at the center of the resonance dip, which is the resonance frequency, the intensity signal at the photodetector will not have a frequency component at the modulation frequency, but rather only at even harmonics of the modulation frequency. Therefore the detection of the resonance frequency is when the intensity signal at the modulation frequency is zero.
The photodetector converts the intensity signal into a voltage signal. If the voltage signal is passed through a demodulator (phase sensitive detector), the output of the demodulator as a function of nominal optical frequency will look like that shown in section (b) of FIG. 2. For an ideal case, the output of the demodulator passes through zero when the nominal optical frequency is exactly at the resonance frequency.
FIGS. 3-1 and 3-2 show an example of resonance tracking electronics that goes with FIG. 1. The CW signal from the photodetector is demodulated at frequency fm. The output of the demodulator is an error signal that crosses through zero when the CW light wave is on a resonance frequency of the ring resonator. The error signal is integrated by an integrator. The output of the integrator controls the laser frequency to the resonator resonance frequency. The CCW control loop is similar, except the output of the integrator goes to a voltage-to-frequency converter, which output is at Δf and is proportional to the rotation rate.
FIG. 4 shows an RFOG with common phase modulation of the laser light before it is split into the two beams that will counter propagate through the coil. There are many different phase modulators that can be used. Some examples are: a fiber wound around a PZT transducer, an integrated optics chip, or a bulk electro-optic modulator.
FIG. 5 shows an RFOG with common frequency modulation of the laser light before it is split into the two beams that will counter propagate through the coil. Some typical ways of modulating the laser frequency is by either modulating the injection current of a diode laser or diode pump laser, or by modulating the length of the laser cavity with a PZT or some type of MEMS device.
There are imperfections in the gyro that will cause an error in detecting the resonance frequency, thus an error in rotation sensing. There are two types of modulator imperfections that can result in rotation sensing errors. One type is modulator intensity modulation. Even though the intended modulation is either cavity length, optical frequency or optical phase, a non-ideal modulator will also generate a modulation of the light intensity which can have a component at the modulation frequency. The unwanted intensity modulation will be detected by the demodulator and interpreted as a signal indicating an off resonance condition. Resonator tracking electronics will then move the laser frequency away from the resonance frequency until the normal resonator intensity signal exactly cancels out the unwanted intensity signal. The deviation away from the resonance frequency results in a rotation sensing error if the unwanted intensity signals are different between the two counter-propagating light waves. However, if the unwanted intensity signals can be made the same, then no rotation sensing error will occur.
Another modulator imperfection that can result in rotation sensing errors is modulation distortion. The modulation distortion can occur at the modulator drive electronics or the modulator. An ideal modulation is a sinusoidal modulation at a single frequency. However, distortion can result in the generation of higher harmonics on the modulation. Even harmonic modulation will result in a resonance detection error which can lead to a rotation sensing error.
One way to reduce or eliminate rotation sensing errors due to modulator imperfections is to employ common modulation of the two counter-propagating light waves. This is done by using the same modulator for both counter-propagating light waves. FIGS. 1, 4 and 5 show various RFOG configurations employing common modulation. By using the same modulator, the resonance detection errors are the same for both the clockwise and counterclockwise directions. Since the rotation measurement is the difference between the detected clockwise and counterclockwise resonance frequencies, a common error will cancel out (common mode rejection) in the rotation measurement.
One drawback to common modulation is the gyro system becomes sensitive to other imperfections associated with optical back-reflection or backscatter within the resonator. Backscattered light will result in two types of errors. One type of error (interference-type) is associated with the optical interference between the backscattered wave and the primary wave that reaches the photodetector. The other type of error (intensity-type) is associated with the intensity of the backscattered wave, which is modulated by the modulation over the resonance dip just like the primary wave.
A method for eliminating the intensity-type error, which is found in the prior art, is to employ independent phase modulation of the counter propagating beams before they enter the resonator. This method is shown in FIG. 6. The frequency of the phase modulation of each beam is set to be different from each other and not harmonically related to each other. This way, the intensity signal of the backscatter light is not at the same frequency as the intensity signal of the primary wave, and can be rejected to a very high degree by a synchronous demodulator employed in the signal processing electronics. The disadvantage of employing independent modulation of the counter-propagating light waves is that modulator imperfections are no longer cancelled out through common mode rejection.
FIG. 7 shows an example of resonance tracking electronics that goes with FIG. 6. The CW signal from the photodetector is demodulated at frequency fm,2. The output of the demodulator is an error signal that crosses through zero when the CW light wave is on a resonance frequency of the ring resonator. The error signal is integrated by an integrator. The output of the integrator controls the laser frequency to the resonator resonance frequency. The CCW control loop is similar, except the signal from the photodetector is demodulated at frequency fm,1 the output of the integrator goes to a voltage-to-frequency converter, which output is at Δf and is proportional to the rotation rate.
There exists a need to have an RFOG configuration that eliminates rotation sensing errors due to modulator imperfections and backscatter.